"What the heck are you doing?" Sweden's prime minister asked on Tuesday (14 August), after up to 100 cars had been set on fire by young, masked men in several south-western Swedish cities during the night.

Social Democrat prime minister Stefan Lofven also called for more resources for the police and tougher criminal punishment during the first big TV debate among party leaders held on Tuesday evening.

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The debate was organised by two of Sweden's largest newspapers, Expressen and Dagens Industri, some three weeks ahead of Sweden's general elections on 9 September.

It was transmitted live from Gothenburg, one of the cities hit hardest by the arson attacks the night before.

"The most important now is to show that society mobilises against crime. It is up to us, the politicians, the police, local authorities, companies and civil society organisations. We need to show that society is always stronger than the criminals," Lofven said.

"These people are limiting other people's freedom, and we won't tolerate it," he said.

Leader of the conservative opposition Moderate Party, Ulf Kristersson, also appealed for more resources, more police officers to be trained, and higher salaries for the forces.

"We can not just say time and again that it is unacceptable and then in practice let it happen," he said.

Two men, aged 16 and 21, were arrested on Tuesday, suspected of arson and instigating riots, most likely organising it via social media, Swedish Television SVT reported. A third was arrested at an airport in Turkey after fleeing the country, according to Swedish police.

But it is not clear who was behind Monday's attacks - only that they could not have come at a more important time in order to influence the political debate.

Jimmie Akesson, leader of the anti-migrant and EU-sceptic Sweden Democrat party did not rule out that the assistance of other EU countries' police may be needed if Swedish police fail to stop the arson attacks on cars.

"If we do not manage to solve it ourselves, we must try to find all possible solutions to stop this development," he told Swedish daily Aftonbladet.

Akesson said it is about a "society that is torn apart because of unsuccessful integration and increased segregation".

"It's an attack on society. This issue needs to be resolved in depth. This government has had four years to solve the problems. They have not succeeded in anything. It's time to replace them," he said.

In June, the Sifo polling institute found 18.5 percent support for the Sweden Democrats, but support dropped to 16.8 percent in August.

Some 23 percent of voters still consider immigration to be the most important issue, but 16 percent have now come to view the environment as the most important, ahead of healthcare at 13 percent.

The drought and forest fires ironically may help Sweden's Green Party to keep its representation in parliament, as it has grown in support over summer from four percent in June to 5.6 percent in August, according to the latest SIFO institute poll, published by Svenska Dagbladet.

The electoral threshold in Sweden is four percent.

The Sweden Democrat's deny there is a link between climate change and the extreme summer temperatures.

Their party leader came under collective attack on the topic from other party leaders in Tuesday's TV debate. "It's embarrassing to have a climate-denier as a party leader," said Centre Party leader, Annie Loof , calling Akesson a "Trump clone".

But following Monday's arson attacks on cars the political focus has swung back to where it suits the Sweden Democrats.

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